Total joint replacement is an effective treatment for relieving pain and restoring function for patients with damaged or degenerative joints. Approximately 500,000 total hip and knee replacements are performed each year in the United States. Although many of the outcomes are successful, there are still significant problems with implant loosening and failure. In fact, 25% of hip replacement surgeries were revisions due to previous implant failure. Surgery to replace these failures is more difficult and costly to perform and has a poorer outcome than the original joint replacement surgery. If fixation is not sufficient, loosening and osteolysis of the implant can occur. To overcome this problem, it is thought that bone implant materials need to stimulate rapid bone regeneration in order to fill in deficient bone and fix the implant firmly with the adjacent bone. The material surface must be able to recruit bone forming cells, such as osteoblasts, such that they can colonize and synthesize new bone tissue.
In order to design better implant materials, it is important to understand the events at the bone-material interface. As mentioned earlier, one of the important challenges is to induce bone growth on the implant surface. The level of bone growth depends on the surface characteristics of the implant. The first event that occurs after the implantation of a biomaterial is the adsorption of proteins from blood and other tissue fluids. Primarily, a hematoma, swelling filled with blood due to a break in the blood vessel, is present between the implant and bone. Cytokines and growth factors stimulate the recruitment of mesenchymal cells which differentiate into osteoblast that are responsible for bone formation. Over time, woven bone matures into lamellar bone which further strengthens the bone-implant interface. Thus, the surface properties play a critical role in long term stability and functionality of the implant.
In an attempt to enhance the stability of endosseous implants, a large number of implant materials and designs have been used. In addition to cement-based prosthetics, much attention in recent years has turned to microinterlocked implants, which have microporous surfaces to allow for the ingrowth of bone. Early work using oxide ceramics showed that a minimum interconnected pore diameter of approximately 100 μm was needed for adequate bone ingrowth (Hulbert et al., J Biomed Mater Res 1972; 6(5):347-74). It was thought that smaller pore sizes allowed incomplete mineralization of the infiltrating tissue. Subsequent use of metallic implants showed bone ingrowth with pore sizes between 50 and 400 μm (Bobyn et al., Clin Orthop Relat. Res 1980(150):263-70). However, recent studies have revealed the possibility that much smaller pores may allow bone ingrowth when presented at high density within metal-oxide substrates. For example, nanoporous Ca—P coatings on implants have shown apposition of human bone growth within 2-3 weeks post surgery (Lee et al., J Biomed Mater Res 2001; 55(3):360-7). Osteoblasts cultured on ceramics of different nm-scale textures also exhibit altered morphologies and growth rates (Boyan et al., Biomaterials 1996, 17(2):137-46; Popat et al., J Orthop Res 2006, 24(4):619-27; Popat et al., Biomaterials 2005, 26(22):4516-22; Swan et al., Biomaterials 2005, 26(14):1969-76; Swan et al., J Biomed Mater Res A 2005, 72(3):288-95; Webster et al., Biomaterials 2004, 25(19):4731-9; Webster et al., J Biomed Mater Res A 2003, 67(3):975-80; Webster et al., Biomaterials 2000, 21(17):1803-10). Nonetheless, there are several problems related to dissolution of nanoscale coatings over time, and cracking and separation from the metallic substrate (Bauer et al., Clin Orthop Relat Res 1994, (298):11-8; and Bloebaum et al., Clin Orthop Relat Res 1994, (298):19-26). These studies point to the importance of developing more robust and flexible nanoscale architectures to enhance the apposition of bone from existing bone surfaces and stimulate new bone formation.
This invention described below addresses these needs, as well as others.